Gay marriage campaigners were celebrating a major victory on Tuesday after a federal appeals court ruled California's same-sex marriage ban unconstitutional. The long-awaited ruling could pave the way for a US supreme court decision on the voter-approved measure known as Proposition 8.

In a 2-1 decision, a three-judge panel of the ninth US circuit court of appeals in San Francisco agreed with a lower court judge who in 2010 declared the ban to be a violation of the civil rights of gay and lesbian people.

"Proposition 8 serves no purpose, and has no effect, other than to lessen the status and human dignity of gays and lesbians in California, and to officially reclassify their relationships and families as inferior to those of opposite-sex couples," wrote Stephen Reinhardt, one of the court's most liberal judges, in Tuesday's ruling.

"Although the constitution permits communities to enact most laws they believe to be desirable, it requires that there be at least a legitimate reason for the passage of a law that treats different classes of people differently," the ruling states.

"There was no such reason that Proposition 8 could have been enacted."

In its ruling, the appeals panel stressed that its decision applies only to California, which allowed gay marriage before Proposition 8, even although it has jurisdiction in nine western states.

"Whether under the Constitution same-sex couples may ever be denied the right to marry, a right that has long been enjoyed by opposite-sex couples, is an important and highly controversial question," the court said. "We need not and do not answer the broader question in this case."

Campaigners and supporters of equal marriage rights described the ruling as monumental and said it put California on a growing list of states that have ended barriers to marriage for gay and lesbian couples.

Evan Wolfson, founder and president of Freedom to Marry said: "Today's powerful court ruling striking down the infamous Prop 8 affirms basic American values, and helps tear down a discriminatory barrier to marriage that benefits no one while making it harder for people to take care of their loved ones.

"The ninth circuit rightly held that a state simply may not take a group of people and shove them outside the law, least of all when it comes to something as important as the commitment and security of marriage."Chad Griffin, president of the American Foundation for Equal Rights (Afer), the group behind the appeal against Proposition 8, said: "The message it sends to young LGBT people, not only here in California but across the country, [is] that you can't strip away a fundamental right."

California voters passed Proposition 8 in 2008, but it was ruled unconstitutional by federal judge Vaughn Walker in 2010. The ban has remained in place since then, because the ninth circuit court put a stay on the Walker ruling pending appeals.

John Eastman, chairman of the National Organisation for Marriage, which supported the ban, said in a statement: "The ninth circuit court of appeals is the most overturned circuit in the country, and Stephen Reinhardt, the author of today's absurd ruling, is the most overturned federal judge in America.

Same-sex couples will not be able to marry in the state until at least after February 28, the deadline for Proposition 8's backers to appeal to a larger panel of the 9th circuit court. If they lose and then appeal to the supreme court, or appeal to the supreme court directly, there will be further delays.

Lawyers for the coalition of religious conservative groups that sponsored the measure said they had not yet decided which course of action they will take. Still, some legal analysts said that the supreme court might not agree to take the case on because of its narrow focus on California.

"Today's ruling is a perfect set-up for this case to be taken by the US supreme court, where I am confident it will be reversed," wrote Eastman.

The case has already been subjected to lengthy delays. Arguments about the constitutional implications of the case were heard by the panel more than a year ago.

But it put off a decision in order to seek guidance from the California supreme court on whether the ban's sponsors had the legal authority to challenge the ruling. The state's attorney general and governor had decided not to appeal it.

In November, the California court gave the ballot measure backers the go-ahead, ruling that the state's citizens' initiative process grants sponsors the right to defend such measures in court even if state officials refuse to do so.

The case was further complicated when lawyers for the coalition of conservative religious groups behind the ballot measure tried to have the trial ruling struck down after it emerged that Walker, the judge who struck down the ban 18 months ago, was in a long-term relationship with another man.

On Tuesday, the panel also said there was no evidence that Walker was biased and should have disclosed before he issued his decision that he was gay. Walker ruled after the first federal trial to examine if the U.S. Constitution guarantees same-sex couples the right to marry,

An estimated 18,000 same-sex couples in California wed during the four-month hiatus before Proposition 8 took effect, according to the Williams Institute on Sexual Orientation and the Law, a thinktank based at the University of California, Los Angeles.

Ted Olson, a former solicitor general and one of the two lawyers who argued against Proposition 8, said the case was about "equality and freedom and dignity and fairness and decency … It is about us. It is about every one of us … Today we are more American because of this decision."

He believed the analysis was crafted so as to give the US supreme court little room to overturn the ruling, because it followed the precedent of earlier supreme court cases, especially Romer v Evans, a Colorado case which similarly concerned the removal of rights already granted to gays and lesbians.

Even without the final imprimatur of the Supreme Court, the ruling would serve as an important precedent in cases across the country, according to Olson, and give those intent on denying same-sex couples the right to marry considerable pause before pushing for new ballot measures or filing legal challenges.

He said opponents of gay marriage now faced an "insurmountable burden" to argue the case back in the other direction.